1. Field of the Invention
The present invention is directed generally to implantable electrode arrays, and more particularly to implantable electrode arrays used to deliver electrical stimulation to the spinal cord.
2. Description of the Related Art
Spinal cord injuries are estimated to afflict over 1.3 million individuals in the United States alone, and paralysis is estimated to affect over 5 million individuals. See “One Degree of Separation: Paralysis and Spinal Cord Injury in the United States,” Christopher and Dana Reeve Foundation (2009). The debilitating nature of paralysis has a profound effect on quality of life, making even partially effective treatments highly desirable goals for the scientific community.
Fortunately, experimental research on animals has shown that some level of recovery of locomotion is possible. In particular, epidural spinal cord stimulation has been shown to induce stepping in rats. See R. M. Ichiyama, G. Courtine, Y. P. Gerasimenko, G. J. Yang, R. Brand, I. Lavrov, H. Zhong, R. Roy, V. R. Edgerton, “Step Training Reinforces Specific Spinal Locomotor Circuitry in Adult Spinal Rats”, J. Neuroscience, vol. 29, pp. 7370-7375 (2008); and R. M. Ichiyama, Y. P. Gerasimenko, H. Zhong, R. R. Roy, V. R. Edgerton, “Hindlimb stepping movements in complete spinal rats induced by epidural spinal cord stimulation,” Neuroscience Letters, vol. 383, issue 3, pp. 339-344 (2005). In these studies, rats were implanted with up to eight wire electrodes. The implanted wire electrodes each extended from a headplug down the neck and to the spinal cord of the rat. During testing, each of the rats was suspended in a jacket such that its hind limbs were positioned on a treadmill. About two weeks after the spinal cord injury, clear stepping patterns were evident when the spinal cord was stimulated. This suggested that the electrical stimulation activated a central pattern generator in the spinal cord.
The following publications provide examples of work related to electrode arrays used to apply electrical stimulation to the spinal cord: D. C. Rodger, W. Li, A. J. Fong, H. Ameri, E. Meng, J. W. Burdick, R. R. Roy, V. Reggie Edgerton, J. D. Weiland, M. S. Humayun, Y. C. Tai, “Flexible microfabricated parylene multielectrode arrays for retinal stimulation and spinal cord field modulation,” Proc. 4th International IEEE-EMBS Special Topic Conference on Microtechnologies in Medicine and Biology, Okinawa, Japan, pp. 31-34 (2006); K. W. Meacham, R. J. Giuly, L. Guo, S. Hochman, S. P. DeWeerth, “A lithographically-patterned, elastic multi-electrode array for surface stimulation of the spinal cord”, Biomedical Microdevices, vol. 10, no. 2, pp 259-269 (2008); and D. C. Rodger, Wen Li, H. Ameri, A. Ray, J. D. Weiland, M. S. Humayun, Y. C. Tai, “Flexible Parylene-based Microelectrode Technology for Intraocular Retinal Prostheses,” Proc. IEEE-NEMS 2006, pp 743-746 (2006).
The publications cited above and other work has led to various designs for high-density electrode arrays to further research, but unfortunately none of these designs has been successfully implanted chronically. A need exists for a chronic implant because chronic implantation is necessary for many applications, such as conducting research, helping a patient move (e.g., step, stand, grip, and the like), improving control of voluntary functions (e.g., voiding the bladder), improving functionality of autonomic processes (e.g., temperature control), and the like. A need also exists for an electrode array assembly configured to more accurately deliver electrical signals to selected locations along the spinal cord. The present application provides these and other advantages as will be apparent from the following detailed description and accompanying figures.